feat: improve JSON error logging for Durable (#298)

* Improve Durable JSON serialization error logging

* Improve Durable JSON serialization error messages

* Add tests

* Remove unused test code

---------

Co-authored-by: hallvictoria <[email protected]>

Author: andystaples

azure/functions/durable_functions.py

@@ -99,10 +99,10 @@ def decode(cls,
             except json.JSONDecodeError:
                 # String failover if the content is not json serializable
                 result = data.value
-            except Exception:
+            except Exception as e:
                 raise ValueError(
                     'activity trigger input must be a string or a '
-                    f'valid json serializable ({data.value})')
+                    f'valid json serializable ({data.value})') from e
         else:
             raise NotImplementedError(
                 f'unsupported activity trigger payload type: {data_type}')
@@ -115,9 +115,9 @@ def encode(cls, obj: typing.Any, *,
         try:
             callback = _durable_functions._serialize_custom_object
             result = json.dumps(obj, default=callback)
-        except TypeError:
+        except TypeError as e:
             raise ValueError(
-                f'activity trigger output must be json serializable ({obj})')
+                f'activity trigger output must be json serializable ({obj})') from e
 
         return meta.Datum(type='json', value=result)
 

tests/test_durable_functions.py

@@ -165,6 +165,19 @@ def test_activity_trigger_encode(self):
                 expected_type=type(datum['output']))
             self.assertEqual(encoded, datum['expected_value'])
 
+    def test_activity_trigger_encode_failure_exception_has_cause(self):
+        class NonEncodable:
+            def __init__(self):
+                self.value = 'foo'
+
+        data = NonEncodable()
+
+        try:
+            ActivityTriggerConverter.encode(data, expected_type=None)
+        except ValueError as e:
+            self.assertIsNotNone(e.__cause__)
+            self.assertIsInstance(e.__cause__, TypeError)
+
     def test_activity_trigger_decode(self):
         # Activity Trigger allow inputs to be any JSON serializables
         # The input values to the trigger should be passed into arguments
@@ -209,6 +222,17 @@ def test_activity_trigger_decode(self):
                 trigger_metadata=None)
             self.assertEqual(decoded, datum['expected_value'])
 
+    def test_activity_trigger_decode_failure_exception_has_cause(self):
+        data = Datum('{"value": "bar"}', 'json')
+
+        try:
+            ActivityTriggerConverter.decode(
+                data=data,
+                trigger_metadata=None)
+        except ValueError as e:
+            self.assertIsNotNone(e.__cause__)
+            self.assertIsInstance(e.__cause__, TypeError)
+
     def test_activity_trigger_has_implicit_return(self):
         self.assertTrue(
             ActivityTriggerConverter.has_implicit_output()